Blow molding techniques are commonly used to produce hollow three-dimensional articles such as containers (for example bottles, etc.), typically from thermoplastics materials which are available as granules, pellets or powders. A blow molding apparatus will typically have a two-piece mold, each mold half having recesses on their inner (facing) surfaces which, when the mold is closed, forming a cavity having the desired shape of the molded article.
A typical blow molding process may include the following sequence of steps:                first softening the granules or powder of thermoplastic material may in a plasticising cylinder;        then extruding the thermoplastic material into a tube or “parison”, which may extend generally vertically from a bottom of an extruder;        surrounding the soft, warm parison with the open mold;        closing the mold to seal the lower end of the parison; and        inflating the parison pneumatically (from its top end) to expand it into the mold cavity;        contact of the expanded parison with the mold cavity may result in rapid cooling of the thermoplastic material.        
FIG. 1 illustrates a basic blow-molding technique 100. A parison 150 of heated plastic material is extruded from and exits the bottom of an extruder. The extruded parison is elongate and tubular, and as it is being extruded it is “pulled” by gravity. The parison is located between two halves 120L and 120R of a mold, the inner surfaces of which have a cavity (dashed lines) in the form of the external surface of desired molded part (such as a bottle). When the parison has sufficient length (it is shown already elongated), the mold closes on the parison. The bottom of the parison is pinched off by the bottom of the mold. A blow pin 114 extends from the extruder into the top of the parison, and the top of the mold seals the parison around the blow pin. Pressurized air blown through the blow pin into the parison expands the parison into the cavity of the mold, conforming thereto. Molding also cools the plastic. The mold is opened, and the resulting molded part is extracted therefrom. A more complete description can be found in Blow Molding, Packaging Technology, 2012, (http://packaging-technology.org/35-blow-molding.html), 26 pages, incorporated by reference herein. See also Extrusion Blow Molding, from “A Guide to Injection Moulding of Plastics”, Prabodh C. Bolur, (http://www.pitfallsinmolding.com/blomolding.html), 10 pages, incorporated by reference herein.
Using blow molding apparatus such as described above, the outside dimensions of the resulting part can be accurately determined, but part wall thickness, and its distribution, depends on the size of the parison and the geometry of the mold. The resulting part wall thickness also depends on the length and weight of the parsion extruded prior to forming the part, because gravity pull thins the wall thickness at the top of the parison as it is pulled by the weight of the already extruded material at the bottom of the parison.